1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a secondary battery, a cap assembly thereof, and a method for easily mounting a safety valve in a safety cap assembly.
2. Description of the Related Art
Unlike the primary battery, the secondary battery may be recharged. Non-aqueous electrolyte secondary batteries of high energy density have recently been developed as high power secondary batteries. Lower power batteries in which one battery cell is made into a battery pack are used as the power source for various portable electronic devices such as cellular phones, laptop computers, and camcorders. Bulk size batteries which are formed by connecting several to tens of the secondary battery cells serially or parallelly can be adequate to power electric vehicles.
Depending on the external shape, secondary batteries maybe classified into different types, for example, square and cylindrical batteries. The secondary battery includes an electrode assembly (jellyroll) in which a positive plate and a negative plate of a long belt shape, and a separator as an insulator interposed therebetween are spiral-wound, or in which a positive plate, a negative plate and a separator are stacked in a multiple layer structure. The electrode assembly is inserted inside a container, and a cap assembly with external terminals is mounted on the container to form the battery.
The cap assembly of such secondary batteries has not only a gasket which seals the container and the external terminals electrically connected to the plates of the electrode assembly, but also a safety device breaks down at a predetermined pressure to discharge gas and to thereby prevent explosion of the batteries.
Although such secondary batteries have a protection circuit to detect excessive current and to cutoff the current, a gas is generated by chemical reaction inside batteries which raises their internal pressure, and sometime batteries might explode and burn when the batteries do not work normally or when an unexpected reaction of batteries happens.
Japanese laid-open 2000-223102 patent application, Japanese laid-open 11-219692 patent application, and Japanese laid-open 11-25935 disclose opening a pressure relief valve which breaks down at a predetermined pressure that is mounted in the cap plate that closes the container to prevent explosions caused by abnormal operation of the batteries.
I have discovered that such technology has a problem because the manufacture is not easy due to the formation of pressure relief valve during the process of making the cap plate so that the uniform operation performance can not be expected.
That is, the pressure relief valve is conventionally formed as an integral structure with the cap plate because the relief valve is made of the same material as the cap plate and is made at the same time when manufacturing the cap plate. For example, a cap plate made of aluminum and the pressure relief valve integrated with the cap plate are manufactured by several pressurizations using progressive dies.
I have found that such a conventional structure however, is difficult to manufacture because the pressure relief valve has a thin layer which must be manufactured at the same time as the cap plate. In addition, the pressure condition to open the valve is different because the thickness of the manufactured valve is not uniform, and an uniform operational performance can not be expected.
Such problems become especially serious for secondary batteries for hybrid electric vehicles (HEV) because the size of secondary batteries for HEVs is bigger than other batteries due to the requirement for high power and bulk size; accordingly, the thickness of the cap plate becomes thicker.
The manufacturing process becomes more difficult because the cap plate should be integrated with the valve, and cannot provide the characteristics necessary for batteries adequate to provide electrical power for HEVs.